leds: leds-mc13783: Prepare driver to support MC13892 LEDs

This patch rewrite driver code to be ready to add support for
MC13892 LEDs and probe from devicetree.

(cooloney@gmail.com: fix one coding style issue when apply this patch)

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Tested-by: Philippe Retornaz <philippe.retornaz@epfl.ch>
Signed-off-by: Bryan Wu <cooloney@gmail.com>